A motor for driving a vehicle and a motor driving unit for driving the motor are mounted on a so-called automobile, an electric vehicle such as a battery forklift, and a hybrid vehicle. The motor driving unit includes a three-phase inverter which converts DC power from a battery into three-phase AC power and supplies the three-phase AC power to a three-phase motor, and a controller which performs driving control of the motor by controlling the inverter. The controller performs the driving control of the motor, based on a motor command rotation frequency (command rotation speed) received from a main controller in a vehicle and a motor actual rotation frequency (actual rotation speed) received from a rotation sensor of the motor. Japanese Unexamined Patent Publication No. 2000-333468 and Japanese Unexamined Patent Publication No. 2000-32603 disclose this type of motor driving unit.
In addition, Japanese Unexamined Patent Publication No. 2000-333468 discloses that switching elements of an entire arm are disconnected all at once in order to protect the switching elements (overvoltage protection) in an inverter from abnormal overvoltage of a DC power supply voltage which is likely to occur during regenerative braking of a motor in an electric vehicle or a hybrid vehicle.
In addition, Japanese Unexamined Patent Publication No. 2000-32603 discloses that a circuit element is protected in a chopper circuit for traveling in the following way. In an electric motor car such as a battery forklift, a battery is disconnected during regenerative braking of a motor for traveling. In a case where an overvoltage is generated in the chopper circuit for traveling due to regenerative energy generated in the motor for traveling, the chopper circuit for traveling is stopped, and power is supplied to a chopper circuit for cargo handling. In this manner, the regenerative energy generated in the motor for traveling is consumed by the motor for cargo handling so as to suppress an overvoltage.
However, if all of the switching elements of the inverter are disconnected during regeneration of the motor, that is, if an output of the inverter is stopped, a regenerative braking force cannot be obtained.
Therefore, the present invention aims to provide a motor driving unit which can compatibly protect an overvoltage of a circuit element and maintain a regenerative braking force, even if an overvoltage state occurs in a DC voltage on an input side of an inverter during regeneration of a motor.